Process of preparing wood pulp



United States Patent PROCESS OF PREPARING WOOD PULP George A. Richter and Robert H. MacClaren, Rochester,

N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application August 31, 1949, Serial No. 113,478

8 Claims. (Cl. 92-11) This application relates to a method of preparing wood pulp in which the wood is treated with aqueous sulfur dioxide, followed by cooking the so-treated wood with green liquor to prepare a chemical woodpulp which is useful for the preparation of cellulose esters.

In the manufacture of kraft pulp the chemicals employed are ordinarily recovered by first evaporating the spent liquor so as to form a mass known as black liquor containing 55-75% of solids, which black liquor is subjected to a smelting or burning, preferably after the addition of some sodium sulfate, which burnt mass is then mixed with water, the resulting product being known as green liquor. This green liquor is then causticized with Ca(OH)2 whereby a white liquor essentially consisting of NaOH and sodium sulfide is obtained and the calcium carbonate formed is removed therefrom. This white liquor is useful for a subsequent kraft cooking operation. The practice of the cansticizing step is costly and in some respects wasteful in that there is loss of material and it is necessary to operate a causticizing system, i. e. a Dorr causticizing system in order to convert the green liquor to white liquor. The elimination of this step would result in considerable economy in the manufacture of kraft pulp. Up to now no method of manufacturing chemical wood pulp is known in which the green liquor can be used as a cooking liquor for the wood, this being the sole cooking liquor. In any previous case where green liquor has been used directly in preparing wood pulp the wood has been first pulped with a well-recognized cooking liquor and the green liquor has only been employed for the further refining of that pulp.

0f the wood pulps which have been considered as useful for the preparation of cellulose esters a sulfite cooking process has been used almost to the exclusion of any of the other wood cooking processes. As a consequence, substantially all of the chemical wood pulp which has been found to be useful as the starting material in preparing cellulose esters has been a sulfite pulp and although kraft pulp has been found to be useful in paper manufacture, its use in preparing cellulose esters has been negligible, if any.

One object of our invention is to provide a method of preparing chemical wood pulp in which green liquor is used as the cooking liquor for the wood. Another object of our invention is to provide a kraft method for the preparation of chemical wood pulp in which the resultant product is eminently suitable for use as the starting material in the preparation of cellulose esters. A further object of our invention is to provide a kraft method of preparing wood pulp in which a causticization treatment in reclaiming spent cooking liquor is unnecessary thereby resulting in considerable economy in the reclaiming of that spent liquor from a previous kraft operation of making wood pulp. Other objects of our invention will appear herein.

We have found that green liquor as ordinarily prepared in the wood pulp industry from the spent liquor from a kraft process is eminently suitable for preparing pulp from wood by a cooking operation if the wood is first subjected to a treatment with aqueous sulfur dioxide such as is described herein. By use of this preliminary heating of the wood in aqueous sulfur dioxide the noncellulosic constituents of the wood are converted to a form which is susceptible to removal by the green liquor when the wood is cooked therewith.

The first step of our invention involves a heating of chips of the wood to be pulped in an aqueous solution mark is passed.

2,694,631 Patented Nov. 16, 1954 "ice restricted to temperatures between approximately C.

and C. as the use of a temperature much above 110 has an adverse elfect upon the later removability of the non-cellulosic constituents of the wood chips. In this operation which is carried out in a closed container or an autoclave the pressure is ordinarily -200 pounds per square inch. After the chips have been treated for the desired time there is little change in their appearance other than in color. The liquid is then drained from the chips and the chips may then be mixed directly with the pulping liquor or, if desired, a Washing with water may be performed. This digestion in S02 liquor appears to so change the non-cellulosic ingredients of the wood that they are readily susceptible to removal by the later cooking with green liquor.

In the second step in which the chips are pulped the cooking liquor may be either green liquor just as it is obtained from a recovery system in a kraft pulp mill or it may be mixed with S02 or with S02 and NaOI-I. These additions may or may not be necessary to return recovered liquor to desired cooking concentration. Further, the S02 may be added if a portion of the NazO present is to be as NazSOs. If desired, instead of using the green liquor from recovery in a kraft pulp mill the cooking liquor used may be a mixture of sodium sulfide and sodium carbonate in aqueous solution, either with or without added S02 and/or NaOI-I. The cooking liquor used Will preferably contain 3-5 pounds of solid in terms of NazO per cubic foot of cooking liquor. A typical green liquor is one in which of the solid content the sodium carbonate is in the order of 70% of the Na20 and the sodium sulfide is in the order of 30% of the NazO. A typical cooking liquor or green liquor which has been modified with S02 would be one in which of the solid material sodium carbonate constituted approximately 40% of the Na2O, sodium sulfide approximately 30% of the Na2O, and sodium sulfite approximately 30% of the Na20. A typical cooking liquor in which green liquor was modified by the addition of S02 and NaOH would have approximately the following analysis as regards its solid content, all in terms of Na20: sodium carbonate 35%, sodium sulfide 30%, sodium sulfite 30%, and NaOH 5%. A small quantity of other sodium-sulfuroxygen compounds such as Na2S203 is also present by virtue of some side reaction of S02 and NazS. In all of the proportions given the analyses are in terms of Na2O as related to the total content of Na20 which is present in the liquor. Of the sodium constituents of the liquor 20-40% thereof should be sodium sulfide, the sodium sulfite should be within the range of 040%, the sodium hydroxide not more than 5%; and the sodium carbonate the remainder, this being on the basis of the total Na20 present, the percentages being on the Na20 basis. There may or may not be impurities which are present such as sulfate, thiosulfate, iron compounds and the like. If green liquor is employed, it is desirable, but not necessary, that the liquor be filtered so that materials which are in suspension therein may be removed prior to the cooking operation. Although this invention is primarily concerned with the economy involved in the use of green liquor as a cooking liquor for preparing wood pulp, if economy is not a consideration, the cooking liquor which is to be employed may be made up by dissolving the sodium compounds listed, in water in proportions within the ranges given.

After the wood chips have been treated with aqueous S02 as mentioned previously, those chips are then cooked in an autoclave with green liquor or modified green liquor or a solution having a comparable composition to form pulp therefrom. This cooking operation is carried out by first heating the mass up to 175 C., with no pressure relief after the 100 C. temperature If the green liquor is used hot and the wood chips are at an elevated temperature, such that their mixture is 100 C., a-time of l-l /zhoursmay-be required to arrive at a temperature of 160-175" C. After the mass has been heated up to this point, the autoclave remains closed and this heat is continued so that the cooking operation takes place over a period of 2-3-hours. The pulp is then dumped and recovered as is.customary in wood pulping operations.

The pulp thus obtained is of value as a starting cellulose in preparing cellulose esters. It is desirable, however, that it first be given a mild bleaching treatment toassure good color in the cellulose ester which is prepared therefrom. This bleaching treatment of the pulp preferably involves the customary 4-step bleaching operation in which the pulp is first treated with chlorine, then with dilute alkali, then with alkaline hypochlorite, and last with a souring reagent such as hydrochloric acid all as described in Richter et a1. Patent No. 1,784,161. For some purposes the intermediate alkali treatment can be eliminated since the tat-cellulose of the unbleached fiber is already in the neighborhood of 9397%. See Richter et a1. Patent No. 1,780,943. The pulp is then thoroughly washed and dried and is ready for xanthation, nitration, acetylation or other conversion. lf de sired, instead of being used for making cellulose esters or other cellulose derivatives, the pulp may be employed in the manufacture of paper in the customary manner.

The'wood which is employed in making pulp in accordance with our invention may be of any of the pulpable woods, such as western hemlock, fir, spruce, pine, tamarack, or, if desired, a non-resinous wood of the types which are recognized at the present time as being suitable for the manufacture of wood pulp by a kraft process. The wood is converted to the form of chips, the usual practice in the wood pulp art being followed in readying the wood for treatment in accordance with our invention. By the preparation of wood pulp as described herein kraft pulps are obtained containing tcellulose on the order of 97% or more and pentosans as low as 1%. The raw stock obtained by this process is easily bleached with an equivalent of 3 or 4% of available chlorine based on the fiber.

The following examples illustrate our invention:

Example 1.-Western hemlock wood chips having a moisture content of 42% were heated in an enclosed container with a 25% aqueous solution of S02, one hour being used in heating the mass up to a temperature of 110 C. and the mass was heated for five hours at that temperature, the pressure developed and maintained being 200 p. s. i. The chips were thoroughly washed and were then mixed with a cooking liquor prepared by adding S02 solution to green liquor in an amount equivalent to /3 of the sodium carbonate therein. The vapors formed during this mixing were absorbed in a second solution containing 4 pounds of NazO per cubic foot in the form of NaOH and the equivalent of 0.02 pound of NazO per pound of wood. The two liquors were mixed giving an approximate concentration as follows:

After placing in the autoclave, the mass was heated up to a temperature of 166 C. which consumed 1 /2 hours. This temperature of approximately 166 C. was maintained for 2 /2 hours. The resulting product was obtained in a yield of 33.4% based on the original wood, the pentosan content thereof was approximately 1.2% and the K number was about 16. The cooking vessel was dumped and the pulp was thoroughly washed and was then refined by first subjecting to aqueous chlorine in a proportion of 4% of chlorine based on the dry weight of the pulp. The pulp was then washed and treated with half percent aqueous NaOH at a temperature of 50 C. After again washing, the pulp was treated with an aqueous solution of sodium hypochlorite and NaOH in the proportions of 3% of sodium hypochlorite (based on pulp) and 1% of NaOH (based on pulp) ata temperatureof45-C. The 'pulp'was'then washed and treated with an aqueous solution of /2% hydrochloric acid as the souring agent. After thoroughly washing, the pulp was dried and was found to have a reflectance of 85.3%. In order to test the susceptibility of the material to acetylation a small 9 g. sample thereof having a moisture content of 4% was actylated by a treatment involving first soaking with 20.5 ml. of acetic acid for one hour at 38 C., adding 48.5 ml. of acetic acid containing 3% of sulfuric acid based on the pulp and tumbling in a rotary tumbler for one hour at 38 C. The mass was then cooled to 17 C. and was mixed in an acetylation vessel with a mixture, having a temperature of 3 C., of 31 ml. of acetic anhydride and 4 ml. of acetic acid containing 4% of sulfuric acid based on the pulp. The mass was thoroughly mixed and the acetylation process took one hour during which time a maximum temperature of 45-50 C. was reached. 30 ml. of 87% acetic acid having a temperature of 5 C. was added, the mass was well mixed and the resulting dope was. observed for haze, viscosity. and color. The dope had a haze value of 16.0 cm., a viscosity of approximately 62 and a color of 200.

Example 2.Wood chips having a moisture content of 42.2% were immersed in aqueous 25% $02 in an enclosed vessel which was heated to a temperature of C. for one hour and then at that temperature for five hours. The pressure which developed was 200 p. s. i. The chips thus treated were washed with water and were placed in a pulping vessel together with a green liquor containing 4 pounds of sodium ion, figured as NazO, per cubic foot of liquor and 0.2 pound of sodium ion figured as NazO per pound original bone dry wood. The sodium ion figured as NazO was divided up in the cooking liquor as follows: NazS 30%, NaOH 10%, NazCOg 60%. The mass was heated up to a temperature of 166 C. over a period of 1% hours and then heated at that temperature for 2 /2 hours. The vessel was dumped and the pulp was thoroughly washed. The pulp gave a yield of 33.3%, had a pentosan content of 1.25% and a K number of 12.4. The pulp was thoroughly washed and subjected to a mild bleaching involving first a treatment with 4% chlorine, then after washing a treatment with /z% NaOH at 50 C., following which, after washing, the pulp was treated with a solution of 3% sodium hypochlorite (based on pulp) and 1% NaOH (based an pulp) at 45 C. After a water washing, the pulp was soured with /'2% aqueous hydrochloric acid and was then thoroughly washed. The pulp obtained exhibited a reflectance of 87.9%. A sample of this pulp after drying was acetylated in the same manner as described in the preceding example. The following values were obtained:

Haze cm. 15.0 Viscosity sec. 75 Color 190 Example 3.-The above example was repeated except that the cooking liquor used had a percentage of NaQCOa Haze cm. 14.5 Viscosity sec. Color Example 4.The preceding example was repeated except that the chips employed had a moisture content of 45.7%. The pulp obtained gave a yield of 33.4%, a pentosan content of 1.18% and a K number of 7.1 The pulp was subjected to a water wash at 60 C. for one hour and was then bleached as described in the preceding example. The pulp after drying was found to have a reflectance of 89.8%. Acetylation of a sample of the pulp was carried out as described in the preceding examples. The following values were obtained:

Haze cm. 13.5 Viscosity see. 50 Color 120 Haze 15.5 Viscosity 43 Color 220 The results compare with the following obtained using acetylation grade cotton linters:

Haze 31.0 Viscosity 46. Color 50 If the procedure in Example 1 were followed except for the wood pulp preparation being a single stage kraft cook, the end product would be of such a character that upon acetylation the acetate results could not be determined because the dope would be an extremely dark, generally unreacted cellulosic mixture.

What we claim is:

1. A method of preparing wood pulp having a high a-cellulose and low pentosan content which comprises heating wood chips with aqueous S02 solutions having a solution concentration of 12 to 35% S02 at 100 to 110 C. for four to ten hours in an enclosed vessel whereby the wood remains unpulped, and subjecting the so-treated wood to a pulping operation with an alkaline pulping liquor essentially consisting of sodium carbonate, sodium sulfide and sodium sulfite, the sodium carbonate being the principal sodium compound, having a concentration of 3-5 lbs. of sodium ion in terms of NazO per cubic foot of liquor under pressure at 160-175 C. for 2-4 hours.

2. A method of preparing wood pulp having a high a-cellulose and low pentosan content which comprises heating wood in comminuted form with aqueous S02 having a solution concentration of 12-35% S02 and less than 0.5 combined S02 at 100-110 C. for 4-10 hours in an enclosed vessel whereby the wood retains its physical form but the binding material therein is rendered more susceptible to removal, subjecting the so-treated wood to a pulping operation with sulfate process green liquor having a concentration of 3-5 pounds of sodium ion in terms of NazO per cubic foot of liquor under pressure at 160-175" C. for 2-4 hours.

3. A method of preparing wood pulp having a high a-cellulose and low pentosan content which comprises heating wood in comminuted form with aqueous S02 having a solution concentration of 12-35% S02 and no combined S02 at 100-110 C. for 4-10 hours in an enclosed vessel whereby the wood retains its physical form but the binding material therein is rendered more susceptible to removal, subjecting the so-treated wood to a pulping operation with sulfate process green liquor having a concentration of 3-5 pounds of sodium ion in terms of Na20 per cubic foot of liquor under pressure at 160-175 C. for 2-4 hours.

4. A method of preparing wood pulp having a high a-cellulose and low pentosan content which comprises heating wood in comminuted form with aqueous S02 having a solution concentration of 1235% S02 and less than 0.5% combined S02 at 100-110 C. for 4-10 hours in an enclosed vessel whereby the wood retains its physical form but the binding material therein is rendered more susceptible to removal, subjecting the so-treated wood to a pulping operation with sulfate process green liquor to which has been added an amount of S02 approximately stoichiometrically equal to about onethird of the sodium carbonate content of the green liquor, this green liquor having a concentration of 3-5 pounds of sodium ion in terms of Naz0 per cubic foot of liquor the treatment being conducted under at 160-175" C. for 2-4 hours.

5. A method of preparing wood pulp having a high a-cellulose and low pentosan content which comprises heating wood in comminuted form with aqueous S02 having a solution concentration of 12-35% S02 and less than 0.5% of combined S02 at 100-110 C. for 4-10 hours in an enclosed vessel whereby the wood retains its physical form but the binding material therein is rendered more susceptible to removal, subjecting the sotreated wood to a pulping operation in sulfate process green liquor to which NaOH has been added to the pressure extent of approximately 15% of the total NazO content of green liquor, the liquor having a concentration of 3-5 pounds of sodium ion in terms of Na20 per cubic foot of liquor under pressure at 160-175 C. for 2-4 hours.

6. A method of preparing Wood pulp having a high a-cellulose and low pentosan content which comprises heating wood in comminuted form with aqueous S02 having a solution concentration of 12-35% of S02 and less than 0.5 of combined S02 at -110 C. for 4-10 hours in an enclosed vessel whereby the wood retains its physical form but the binding material therein is rendered more susceptible to removal, subjecting the sotreated wood to a pulping operation with sulfate process green liquor to which has been added an amount of S02 approximately stoichiometrically equal to about onethird of the sodium carbonate present in the green liquor and to which was also added sufiicient NaOH to approximate 5% of the total Na20 content of the green liquor, the liquor having a concentration of 3-5 pounds of sodium ion in terms of NazO per cubic foot of liquor the treatment being conducted under pressure at 160-175 C. for 2-4 hours.

7. A method of preparing wood pulp having a high u-cellulose and low pentosan content which comprises heating wood chips in aqueous S02 of 25% concentration to a temperature of C. in an enclosed vessel and maintaining at that temperature and pressure for 5 hours whereby the wood retains its physical form but the binding material therein is rendered more susceptible to removal, cooking the so-treated wood in sulfate process green liquor to which S02 has been added, which liquor has a concentration of 4 pounds of sodium ion in terms of NazO per cubic foot of liquor, then heating the mass to a temperature of approximately 166 C. and maintaining at that temperature for approximately two and one-half hours.

8. A method of preparing wood pulp having a high lx-cellulose and low pentosan content which comprises heating wood chips in aqueous S02 of 25 concentration to a temperature of 110 C. in an enclosed vessel and maintaining at that temperature and pressure for 5 hours, washing the so-treated wood and cooking it in sulfate process green liquor to which S02 has been added, which liquor contains 4 pounds of sodium ion in terms of Na20 per cubic foot of liquor, then heating to a temperature of approximately 166 C. and maintaining at that temperature for two and one-half hours, then treating the so-formed pulp in sequence of refining steps as follows:

1. aqueous chlorine 2. aqueous sodium hydroxide of approximately /2% concentration 3. alkaline sodium hypochlorite followed by treatment with acid to remove mineral constituents therefrom, washing and drying.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,722,993 Bradley et a1 Aug. 6, 1929 1,787,954 Richter Jan. 6, 1931 1,804,967 Wolf et a1. May 12, 1931 1,838,326 Richter Dec. 29, 1931 1,859,847 Rue et a1. May 24, 1932 1,918,250 Roza July 18, 1933 1,921,539 Richter Aug. 8, 1933 1,931,575 Roza Oct. 24, 1933 2,065,396 Richter Dec. 12, 1936 2,542,801 Roza Feb. 20, 1951 OTHER REFERENCES Chemical Abstracts, vol. 35, pp. 1989, 1990 (1941).

Chemical Abstracts, vol. 39, pp. 413, 414 (1945).

Manufacture of Pulp and Paper, 3d Ed., vol. III, Sec. 5, pp. 4, 6 and 193-196 (1937), published by McGraw-Hill, New York.

Chidester et al., Paper Trade Journal, February 9, 1939, pp. 31, 32.

Handbook of Chemistry and Physics, 28th Edition, p. 1362 (1944), published by Chemical Rubber Pub. Co., Cleveland. 

1. A METHOD OF PREPARING WOOD PULP HAVING A HIGH A-CELLULOSE AND LOW PENTOSAN CONTENT WHICH COMPRISES HEATING WOOD CHIPS WITH AQUEOUS SO2 SOLUTIONS HAVING A SOLUTION CONCENTRATION 12 TO 35% SO2 AT 100 TO 110* C. FOR FOUR TO TEN HOURS IN AN ENCLOSED VESSEL WHEREBY THE WOOD REMAINS UNPULPED, AND SUBJECTING THE SO-TREATED WOOD TO A PULPING OPERATION WITH AN ALKALINE PULPING LIQUOR ESSENTIALLY CONSISTING OF SODIUM CARBONATE, SODIUM SULFIDE AND SODIUM SULFITE, THE SODIUM CARBONATE BEING THE PRINCIPAL SODIUM COMPOUND, HAVING A CONCENTRATION OF 3-5 LBS. OF SODIUM ION IN TERMS OF NA2O PER CUBIC FOOT OF LIQUOR UNDER PRESSURE AT 160-175* C. FOR 2-4 HOURS. 